Question

how does atomic size vary in along the period and down the group​

Answer 1

Atomic radius of the elements generally decreases from left to the right in a period because on moving from left to right in aperiod the nuclear charge gradually increases by one unit & one electron is also added in the electron shell.Due to this the electrons get attracted more & more towards the nucleus.consequently
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Answer 2

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Down a group

As the electron enters a new shell, the distance between the nucleus and the outermost electron increases. Thus, atomic size increases down a group.

2. Along a period

As there is no new shell appearing along a period, the distance between the nucleus and the outermost electron remains same but due to attraction between the nucleus and the electron, the distance decreases. Thus, the atomic size decreases along a period.

But, an exceptional case is there of noble gases as they have larger atomic size as compared to the elements in that period because of negligible Van der Waals forces of attraction.

Hope it will help